Method for path-seacher scheduling

ABSTRACT

The invention describes a method for adaptively scheduling activation of a path-searcher for a mobile terminal ( 1 ) operative in a telecommunication system. The mobile terminal ( 1 ) is capable of receiving multi-path signals ( 3, 4 ) originating from a scattered signal from a transmitter of at least one base station ( 2 ) in said system. The time lag between consecutive path-searcher activations is according to the method determined based on a value of the power delay profile discrepancy between two consecutive power delay profiles. The power delay profiles are derived from at least a subset of the powers of signals ( 3, 4 ) received at different delays during the path-searcher activation.

FIELD OF THE INVENTION

The present invention relates to a method for scheduling a path-searcherin a communication system, wherein signals are transmitted from atransmitter to a receiver. More specifically, the communication systemcomprises at least one base station and one mobile communicationterminal, in which system signals comprise waveforms and wherein areceived or preprocessed signal is sampled. According to the method ofthe invention mutual signal delays are utilized for scheduling of apath-searcher of the terminal. The signals originate from one or severaltransmitters, which signals have propagated along several differentpaths.

The invention also relates to a mobile communication terminal adapted toadaptively schedule path-searcher activations depending on thediscrepancy between power profiles of consecutive path-searcheractivations.

DESCRIPTION OF THE PRIOR ART

In a telecommunication system, after a signal has left the transmitterit is scattered into several parts as it propagates. Therefore, severalcopies of the signal arrive at the receiver at slightly different pointsof times, as the signal has propagated with the same speed alongdifferent paths. The signal scattering varies depending on theenvironment the signal has to pass from the transmitter to the receiver,such as buildings and mountains etc. Examples of a mobile communicationterminal are a mobile telephone, a pager, a communicator, i.e. anelectronic organizer, a smartphone or the like.

In WCDMA (Wide band Call Division Multiple Access) and othercommunication techniques multi-path interference due to signalscattering is a common problem, as indicated above. Multi-pathinterference is caused by the broadcast signal travelling over differentpaths to reach the receiver. The receiver then has to recover the signalcombined with echoes of varying amplitude and phase. This results in twotypes of interference;

Inter-chip interference: The reflected signals are delayed long enoughthat successive chips in the demodulated signals overlap, creatinguncertainty in the data.

Selective fading: The reflected signals are delayed long enough thatthey are randomly out of phase, and add destructively to the signal,causing it to fade.

To combat interference, the multi-path signals may be detected,processed and added to the desired signal for maximizing it. To find andidentify multi-path signal delays the mobile terminal comprises apath-searcher. The path-searcher is run to find multi-path signal delaysfrom a transmitter(s) at possible delays from the activation of thepath-searcher. This is performed by first multiplying the receivedsignal at a certain delay with the scrambling and channelization codesfor a signal at a certain delay to derive pilot symbols of the receivedsignal. Then the pilot symbols are multiplied with their complexconjugate and summed over a given number of symbols, and the result issquared. A given number of squares are then averaged, which is a measureof the received power and noise for the delay. This is repeated for agiven number of delays, which constitute the delay window size. Thepowers of the delays of a delay window form a power delay profile. Thedelays having the largest powers are then chosen among the powers of thepower profile that give the largest power for the desired signal. Oncethe delays have been located, despreading of the received signal for thechosen delays can be performed, followed by decoding of the sent bitstream.

In the following, by delay is meant the signal received by the receiverat a given time delay from the activation of the path-searcher, and thedelay power is meant the power of the signal detected at said timedelay. The delay location may constitute of pure noise or a scatteredsignal from a transmitter. Normally, the power from noise is well belowthe corresponding power for a delayed transmitted signal. Further, bymulti-path signals is meant broadcast signals sent from a transmitterand received at any delay of the path-searcher activation.

The number and location in amplitude and phase of the multi-path signalsreceived by the receiver are a function of the speed of the mobilecommunication terminal. The faster the mobile communication terminalmoves in relation to the base station the more frequently the multi-pathsignals emerge and vanish, and hence the more frequently thepath-searcher has to be activated for following the power profile. Themore frequent the path-searcher activations become, the larger the powerconsumption is going to be. This is a problem as low power consumptionis essential for a mobile terminal.

A straightforward solution of the scheduling problem is to schedule thepath-searcher activations according to a fixed pattern. However, thisrequires that the fixed pattern is chosen according to the worstpossible case, which could be a considerably waste of power consumption.Therefore, it is preferred to schedule the path-searcher activationsadaptively depending on the speed of the mobile terminal. In the knownprior art, the speed of the mobile terminal is estimated by estimatingthe Doppler frequency of the received signals. However, as the signalcontains a lot of noise, the estimate of the speed of the terminal isnot very robust, which makes it difficult to use said estimate toschedule the path-searcher activations. Also, clock drift between thebase station and the mobile terminal will not be found using the knownprior art, causing a further degradation of the activations of thepath-searcher.

SUMMARY OF THE INVENTION

The object of the invention is to provide a method for decreasing thepower consumption of a mobile communication terminal operative in atelecommunication system. More precisely, the object of the invention isto provide a method for decreasing the power consumption of apath-searcher of a receiver of the mobile terminal. Also, it is anobject of the invention to provide a method, with which it is possibleto follow the movement of a power profile of a path-searcher.

The above objects is according to the invention achieved by a method,which provides the possibility to adaptively scheduling path-searcheractivations based on the value of the power delay profile discrepancybetween power delay profiles of consecutive path-searcher activations. Apower delay profile are determined for each path-searcher activation,which profiles comprise at least the powers of a subset of the powers ofthe signals received at different delays during each path-searcheractivation. By utilizing the existing power profiles, the value of thepower delay profile discrepancy of two consecutive power delay profilesis determined, which is an indication of the speed of the mobileterminal relatively to the at least one base station. A large powerdelay profile discrepancy is an indication of a fast movement of themobile terminal, and the time lag between consecutive path-searcheractivations may be decreased and vice verse.

Another object of the invention is to provide a mobile communicationterminal, which has decreased power consumption. Also, it is an objectof the invention to provide a mobile communication terminal, which isadapted to follow the power delay profile of a path-searcher withoutestimating the velocity of the mobile communication terminal relativelyto a base station.

A mobile terminal adapted to adaptively schedule activation of apath-searcher of the mobile terminal achieves the above objects.Further, the mobile terminal is adapted to derive power delay profilesof activations of the path-searcher of the receiver of the mobilecommunication terminal. Each profile is based on at least a subset ofthe delay powers detected during the path-searcher activation. Finally,the mobile terminal is adapted to determine a value of the power delayprofile discrepancy between two consecutive power delay profiles. Saidvalue indicates the need of increasing or decreasing the time lag to thenext the next path-searcher activation, which time lag is computed basedon a penalty function penalizing large and rewarding low power delayprofile discrepancies.

The method and the mobile terminal of the invention has the advantage ofdecreasing the power consumption relatively to path-searchers known inthe art due to the adaptable time lag between path-searcher activations.Also, using the method according to the invention compensation for clockdrift between the base station and the mobile terminal is provided.Finally, the inventive method also provides the possibility to robustlyand adaptively schedule the path-searcher without actually estimatingthe velocity of the mobile communication terminal.

Further preferred features of the invention are defined in the dependentclaims.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, preferred and alternative embodiments will bedescribed in more detail with reference to the enclosed drawings, inwhich:

FIG. 1A illustrates a mobile communication terminal adapted according tothe invention to receive multi-path signals from a base station andschedule a path-searcher accordingly;

FIG. 1B is a block diagram showing components of the mobile terminal ofFIG. 1A;

FIG. 2 is a flowchart of the steps of a first embodiment of the methodaccording to the invention;

FIG. 3 is an exemplifying penalty function used to derive the time lagto the next path-searcher activation;

FIG. 4 is a flowchart of the steps of a second embodiment of the methodaccording to the invention;

FIG. 5 is a flowchart of the steps of a third embodiment of the methodaccording to the invention;

FIG. 6 is tables illustrating the ranking of the third embodiment; and

FIG. 7 is a flowchart of the steps of a fourth embodiment of the methodaccording to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

A mobile terminal 1 will in operation in a telecommunication system,such as a wide band call division multiple access (WCDMA) system,experience multi-path signals. A signal from a transmitter of at leastone base station 2 will propagate to the mobile terminal 1 throughdifferent paths due to signal scattering. For example, the signal maytake a direct path 3 from the base station 2 to the mobile terminal 1,or an indirect path 4, wherein the signal is reflected one or severaltimes on e.g. a building 5 or a mountain. Due to the density of objectsreflecting the scattered signal, it will take more or less time for themulti-path signals to arrive at the mobile terminal 1. Consequently, themobile terminal 1 will receive copies of the original signal from thebase station 2 having different delays, i.e. multi-path signals. Toderive a signal having sufficient power, a receiver 10 of the mobileterminal 1 comprises a path-searcher 11 to find multi-path signalsoriginating from the same signal. According to the invention there isprovided a method for adaptively scheduling activations of thepath-searcher 11, which will be explained in the following.

As is readily understood, as the mobile communication terminal 1 isadapted to communicate with e.g. the base station 2, the mobilecommunication terminal 1 also has an complete RF chain for communicatingwith said network 21. However, a detailed disclosure of the RF chain isnot necessary for the understanding of the invention and will thereforenot be further described herein.

If the scheduling of the path-searcher 11 should be executed without theworst case scenario, it is preferred to adapt the time lag between twoconsecutive activations based on variations of the power of detectedsignals at different delays of the path-searcher 11 activation. Roughlyspeaking, the faster the mobile terminal 1 is travelling, the faster themulti-path signals will drift in time and the more likely it may happenthat new paths emerge or old paths vanish. However, each path-searcheractivation increases the total power consumption of the mobile terminal,and hence it is preferred to run the path-searcher 11 with the longestpossible time lags.

The method according to the invention provides the possibility to runthe path-searcher 11 with adaptable time lags between two consecutivepath-searcher activations. According to the method, the delay powers oftwo consecutive path-searcher activations, i.e. the power profiles, willbe compared. Depending on the discrepancy between consecutive powerdelay profiles the time lag to the next path-searcher activation isincreased or decreased. The duration of the path-searcher activationconstitutes a delay window size, during which the power of a signalreceived by the receiver 10 of the mobile communication terminal 1 ismeasured at a number of consecutive delays, which constitute the powerprofile, from the activation of the path-searcher 11. As indicatedabove, the signal received at each delay may constitute of noise or amulti-path signal. The powers of at least a subset of the delays will beaccumulated for each path-searcher activation. If the discrepancy of thepowers of two consecutive power delay profiles is large, the time lagbetween consecutive path-searcher activations will be decreased and viceversa. Consequently, the time lag between two consecutive path-searcheractivations is inverse proportional to the power delay profilediscrepancy.

For convenience, the power detected at a certain delay during thepath-searcher activation will in the following be referred to as thedelay power. As the number of possible delays are determined based onthe possibility that a multi-path signal might be received by thereceiver 10, delay powers may constitute of the power of noise or anactual multi-path signal.

To compute the power of a given delay, the mobile terminal 1 receives asignal, which is first multiplied with the scrambling and channelizationcodes and then added over the length of the channelization code toderive a pilot symbol. Then, the sum of the derived-pilot symbolsmultiplied with the conjugate of the pilot symbol is computed over agiven number of symbols, and the result is squared. A given number ofsquares are then averaged, which is a measure of the received power andnoise for that delay. This is repeated for each delay during theactivation of the path-searcher 11.

The powers of the delays of a path-searcher activation constitutes thepower delay profile, which when derived is stored in the memory 12 ofthe mobile communication terminal 1 to be used according to the methodof the invention.

In the following, different embodiments will be described with referenceto the accompanying drawings. According to the invention, the number ofpossible multi-path signals during one delay window is denoted N_(p) andis given by the path-searcher 11. Both the delay window size and thenumber of possible delays may be set differently depending on differentdesigns and has to be thoroughly tested and evaluated in each specificcase.

The powers of each possible delay are denoted by p_(i), where i=l, . . ., N_(p). A power delay profile p is the sum of the powers of each pathin the delay window received during an activation of the path-searcher11. In the following, the power delay profiles described in thefollowing are normalized for statistical and evaluation purposes, if nototherwise stated, i.e. ${\sum\limits_{i = 1}^{N_{P}}p_{i}} = 1.$

The delays are indexed by the variable i. The time delay between twoadjacent possible delays indexed by i and i+1 could be e.g. a chip.However, there are other possible time delays, such as a ¼ chip, e.g.depending on the required resolution of the path-searcher 11.

In a first embodiment, the current power delay profile is denoted byp^((n)), where n>=1, and the previous power delay profile p^((n−1)),each comprising a number of delay powers p_(i). Consequently, anestimate of the first power delay profile is denoted by p⁽⁰⁾. Theminimum and maximum time lag to the next path-searcher activation isdenoted by τ_(min) and τ_(max), respectively. Examples of values ofthese variables are τ_(min)=70 ms and τ_(max)=500 ms. However, thespecific values of τ_(min) and τ_(max) may be set differently and haveto be thoroughly tested and evaluated in each individual case. τ^((n))is the size of the time lag to the next path-searcher activation, whichis adaptable as set out above.

The first embodiment of the scheduling of activation of thepath-searcher 11 will now be explained with reference to FIG. 2. In afirst step 100, the current power delay profile is derived by thepath-searcher 11 of the receiver 10. The current power delay profile isstored in the memory 12 to be used as the previous delay power profilein a subsequent scheduling of the path-searcher 11.

Then in step 101 it is decided whether a previous delay power profile isretrievable. If not, the scheduling of the path-searcher activation isin step 102 initialized by setting Δ⁽⁰⁾ = 1, Δ_(filt)⁽⁰⁾ = 1,and choosing λ≧0. This is e.g. the case when only the first delay powerprofile p⁽⁰⁾ has been derived. Here, Δ^((n)) is an intermediary powerdelay profile discrepancy variable, Δ_(filt)^((n))is a filtered power delay profile discrepancy variable, and λ is afilter parameter. The filter parameter λ may be chosen differentlydepending on the path-searcher implementation, e.g. the noise levelaccepted and the speed of the filter. The scheduling proceeds from step102 to step 105, wherein the time lag between two consecutivepath-searcher activations, τ^((n)), will initially be set to τ_(min),i.e. the worst case scenario, as will be further explained below. Thisinitialization only has to be made for the first scheduling of thepath-searcher 11, e.g. when the mobile terminal 1 is switched on.Therefore, if Δ_(filt)⁽⁰⁾ = 1is set in step 102, the time lag is set accordingly, and the schedulingis returned to step 100

If it is decided in step 101 that a previous delay power profile isretrievable, in step 103, the value of the power delay profilediscrepancy between the current and the previous power delay profilesp^((n)) and p^((n−1)), respectively, are computed based on the currentand the previous power delay profiles of two consecutive path-searcheractivations and put in the power delay profile discrepancy variableaccording to the following:${\Delta^{(n)} = {\frac{1}{2}{\sum\limits_{i = 1}^{N_{p}}{{p_{i}^{(n)} - p_{i}^{({n - 1})}}}}}},{n \geq 1},{0 \leq \Delta^{(n)} \leq 1}$Consequently, in the first embodiment the value of the power delayprofile discrepancy is based on the actual difference between twoconsecutive power delay profiles.

Then, in step 104 the value of the power discrepancy variable isfiltered to stabilize the noise and get a filtered power delay profilediscrepancy variable Δ_(filt)^((n))by using the following function:Δ_(filt)^((n)) = λ  Δ_(filt)^((n − 1)) + (1 − λ)Δ^((n)).

As is realized, also the value of the filtered power delay profilediscrepancy variable is a real number between zero and one.

Finally, at step 105 the time lag to the next path-searcher activation,τ^((n)), is given by: τ^((n)) = f(Δ_(filt)^((n)))The function f is an arbitrary penalty function such that:ƒ(0)=τ_(max) and ƒ(1)=τ_(min).

The penalty function f may be set differently depending on userpreferences and wanted characteristics of the time lag to the nextpath-searcher activation. Any penalty function penalizing large powerdelay profile discrepancy values and rewarding low power delay profilediscrepancy values may be utilized. When the time lag has been computedin step 105, the scheduling is returned to step 100, where thepath-searcher is activated with the computed time lag τ^((n)).

An example of the penalty function f is shown in FIG. 3, which is apiece-wise linear function. There will always be some amount of noise,e.g. noise peaks detected as multi-path signals, in the estimates of thefiltered power delay discrepancy profile, which means that even thoughthe multi-path signals never move in phase and amplitude, there willnever be a Δ_(filt)^((n))equal to zero. Because of this, the time lag to the next path-searcheractivation should be set to τ_(max) if Δ_(filt)^((n))is smaller than a first threshold value δ₁. As is understood, also thisvariable may be set differently in each individual case depending on thespecific path-searcher implementation. Similarly, the interpretation ofΔ_(filt) ^((n)) being larger than a second threshold value δ₂ is thatthere is a 100  Δ_(filt)^((n))%power discrepancy between the two consecutive power delay profilesp^((n)) and p^((n−1)), respectively. In the function f of FIG. 3, thetime lag between path-searcher activations is set to τ_(min) when thepower discrepancy between two consecutive power delay profiles is largerthan δ₂, which may be set differently in each individual case inaccordance with δ₁.

A second embodiment will now be described, by which the invention isfurther improved in that the memory capacity required will be decreased.

In the second embodiment, only a subset of the powers p_(i) of thederived power delay profile is selected and denoted by {circumflex over(p)}_(i), j=1, . . . , {circumflex over (N)}_(p) for deriving a reducedpower delay profile. In this embodiment, the delay powers are indexed byj and correspond to a particular index i in the first embodiment. Thepowers of the multi-path signals may e.g. be chosen as the {circumflexover (N)}_(p) paths with the largest powers. Consequently, only theselected powers have to be stored in the memory 12 for the followingprocessing for the scheduling of the path-searcher 11. As in the firstembodiment, the selected powers are normalized for processing purposes,i.e.: ${\sum\limits_{j = 1}^{{\hat{N}}_{p}}{\hat{p}}_{j}} = 1.$

The steps of the method according to the second embodiment will now bedescribed with reference to FIG. 4. Some of the variables of the secondembodiment correspond to the like variables of the first embodiment,e.g. τ_(min) and τ_(max), and should have the same meaning. Therefore,they will not be described again in relation to the second embodiment.

In a first step 200, the path-searcher 11 of the receiver 10 is run toderive the current power delay profile. Further, to form a reduced powerdelay profile the delay powers are selected as set out above and storedin the memory 12 to be used as the reduced previous delay power profilein a subsequent scheduling of the path-searcher 11.

Then in step 201 it is decided whether a reduced previous power delayprofile is retrievable. If not, the scheduling of the path-searcheractivation is in step 202 initialized by settingΔ⁽⁰⁾ = 1, Δ_(filt)⁽⁰⁾ = 1,and chose λ≧0, in correspondence with the first embodiment. Then thescheduling proceeds from step 202 to step 205, wherein the time lagτ^((n)) to the next path-searcher activation will be set to τ_(min).

If is decided in step 201 that a reduced previous power delay profile isretrievable, in step 203 the value of the power delay discrepancybetween the current and the previous reduced power delay profiles{circumflex over (p)}^((n)) and {circumflex over (p)}^((n−1)),respectively, is computed using the reduced set of power delay profiles.In this embodiment, the delays corresponding to {circumflex over(p)}_(j) ^((n)) that are at the most q chips away from an old delay{circumflex over (p)}_(j) ^((n−1)) of the previous reduced power delayprofile are extracted. q is dependent of the resolution of path-searcherimplementation, i.e. the time delays between two adjacent delays asdiscussed above. The extracted remaining delays are enumerated as{circumflex over (p)}_(j) _(k) ^((n)), k=1, . . . , {circumflex over(N)}′_(p), where {circumflex over (N)}′_(p)<={circumflex over (N)}_(p).

The value of the power delay profile discrepancy between the current andthe previous reduced power delay profile is computed as:$\Delta^{(n)} = {\sum\limits_{j = 1}^{{\hat{N}}_{p}}{{\hat{p}}_{j_{k}}^{(n)}.}}$Thus, in the second embodiment, the value of the power delay profilediscrepancy Δ^((n)) may be looked upon as the sum of the powers of newdelays which has a considerably power contribution (not noise), i.e. themulti-path signals present in the current reduced power delay profilebut not in the previous.

In step 204, the power delay profile discrepancy is filtered in the sameway as in the first embodiment, that is:Δ_(filt)^((n)) = λ  Δ_(filt)^((n − 1)) + (1 − λ)Δ^((n)).

Once again, the filtered power delay profile discrepancy variableΔ_(filt)^((n))is a real number between zero and one.

Finally, in step 205 the time lag τ^((n)) to the next path-searcheractivation is computed using the penalty function f, i.e.:τ^((n)) = f(Δ_(filt)^((n))).

Also in this embodiment, f is an arbitrary penalty function, such that:ƒ(0)=τ_(max) and ƒ(1)=τ_(min).

The function f may be chosen differently in each individual casedepending on the path-searcher implementation, e.g. according to theexample shown in FIG. 3. When the time lag has been computed in step205, the scheduling is returned to step 200, where the path-searcher isactivated with the computed time lag τ^((n)).

As in the first embodiment, the filtering in step 204 is performed toremove some of the noise inherent in the estimate. Alternatively, thepower delay profile p or the reduced power delay profile {circumflexover (p)} may be filtered in step 200 or 203 to remove some of the noiseinherent in the estimate. As is obvious, the same may be applied also inthe first embodiment.

A third embodiment of the method according to the invention will now dedescribed with reference to FIG. 5 and 6. In the third embodiment, areduced set of power delay profiles {circumflex over (p)}^((n)) and{circumflex over (p)}^((n−1)) will be utilized, as in the secondembodiment. However, as should be noted it is equally possible toutilize complete power delay profiles as in the first embodiment. In afirst step 300 of FIG. 5, the path-searcher 11 of the receiver 10 is runto derive the current power delay profile. Also, the delay powersreceived during the current path-searcher activation are first selected,e.g. as the {circumflex over (N)}_(p) paths with the largest powers.However, other selections are also possible. Also, each selected power{circumflex over (p)}_(j) ^((n)) is ranked and given a ranking weightw_(j) ^(n) where j=1 . . . {circumflex over (N)}_(p), from 1 to{circumflex over (N)}_(p). Other ranking weights are equally possiblewithin the scope of the invention. The ranking of said selected powersare illustrated in FIG. 6, wherein table 1 comprises a ranked previouspower delay profile and table 2 a ranked current power delay profile,respectively. Also, said tables comprise the delay number of each delayfor identification of a specific delay. Each ranked profile is stored ina memory of the mobile terminal 1 to be processed by e.g. a CPU of saidterminal 1. All delays not comprised in each ranked power delay profileare given the weight zero. Consequently; all delays not having a powerover a certain threshold, e.g. the noise floor, do not has to be stored,which saves memory capacity. The ranked current delay power profile isstored in the memory 12 to be used as the ranked previous delay powerprofile in a subsequent scheduling of the path-searcher 11.

Then in step 301 it is decided whether a reduced previous delay powerprofile is retrievable.

If the answer in step 301 is no, then in step 302 of FIG. 5 thepath-searcher scheduling is initialized by settingΔ⁽⁰⁾ = 1, Δ_(filt)⁽⁰⁾ = 1,and chose λ≧0, corresponding to the first and second embodiments. Thenthe scheduling proceeds to step 305, wherein the time lag τ^((n)) to thenext path-searcher activation will be set to τ_(min).

If the it decided in step 301 that a ranked previous delay power profileis retrievable, the scheduling proceeds to step 303 for deriving thevalue of the power delay profile discrepancy between the ranked currentand previous power delay profiles. The value of the power delay profilediscrepancy is given by:${\Delta^{(n)} = {\beta^{- 1}{\sum\limits_{j = 1}^{{\hat{N}}_{p}}{{w_{j}^{n} - w_{{Per}{(j)}}^{n - 1}}}}}},{\beta = {{{\hat{N}}_{p} + \ldots + 2 + 1} = {\frac{{\hat{N}}_{p}( {{\hat{N}}_{p} + 1} )}{2}.}}}$

The delay j of the current ranked power delay profile corresponds to thedelay having index Per(j) of the previous ranked power delay profile,i.e. delays of the current and the previous ranked power delay profileshaving the same time delay. Consequently, the corresponding powerspresent in the current and previous ranked power delay profiles areidentified and compared to identify whether the power of a previousdelay has changed or not, i.e. if the mobile station has moved. In FIG.5 this is e.g. seen by delay number 4, which has the highest ranking intable 1, but has the second ranking in table 2. Consequently, the mobileterminal 1 has been moved during the time lag between the previous andthe current path-searcher activations in this example. Therefore, thecontribution of delay number 4 is added to the power delay profilediscrepancy variable. As should be noted, ranked current and previouspower delay profiles may equally well be compared according to otherschemes than above, e.g. to powers having the same delay plus/minus someadditional time delay.

At step 304, the value of the power delay profile discrepancy isfiltered according to the same principles as set out above to derivevalue of the filtered power delay profile discrepancy variable:Δ_(filt)^((n)) = λ  Δ_(filt)^((n − 1)) + (1 − λ)Δ^((n)).

Finally, in a last step 305 the time lag τ^((n)) to the nextpath-searcher activation is computed as: τ^((n)) = f(Δ_(filt)^((n))).

Also in this embodiment f is an arbitrary penalty function, such that:ƒ(0)=τ_(max) and f(1)=τ_(min).

The penalty function is, as in the previous embodiments, an arbitraryfunction chosen depending on the specific path-searcher implementation.The function shown in FIG. 3 and explained above is one example of thefunction f also for the third embodiment. When the time lag τ^((n)) iscomputed in step 305, the scheduling is returned to step 300, where thepath-searcher is activated with the computed time lag τ^((n)).

A fourth embodiment of the method according to the method of theinvention will now be described with reference to FIG. 7. In the fourthembodiment; complete or reduced power delay profiles may be utilized.For convenience, this embodiment will be explained with reference tocomplete current and previous power delay profiles p^((n)) andp^((n−1)), respectively, as in the first embodiment. In a first step 401the path-searcher 11 of the receiver 10 is run to derive the currentpower delay profile, which is stored in the memory 12 to be used as theprevious delay power profile in a subsequent scheduling of thepath-searcher 11.

Then in step 401, it is decided whether a reduced previous delay powerprofile is retrievable. If not, the scheduling of the path-searcheractivation is in step 402 initialized by settingΔ⁽⁰⁾ = 1,  Δ_(filt)^((n)) = 1,and chose λ≧0, in correspondence with the above embodiments. Then, thescheduling proceeds to step 405, wherein the time lag τ^((n)) to thenext path-searcher activation will be set to τ_(min).

As before, the current and previous power delay profiles used in thefollowing are normalized, i.a.:${{\sum\limits_{i = 1}^{N_{p}}p_{i}^{n}} = 1},\quad{{\sum\limits_{i = 1}^{N_{p}}p_{i}^{n - 1}} = 1.}$

If it is decided in step 401 that a previous power delay profile isretrievable, in step 403 an intermediate discrepancy function isutilized to compute the power delay profile discrepancy. Theintermediate discrepancy function is used to determine whether the powerof a specific delay has changed substantially from the previous to thecurrent power delay profile. The discrepancy function is as follows:${f( {p_{i}^{n},p_{i}^{n - 1}} )} = \{ \begin{matrix}{\frac{1}{2}( {p_{i}^{n} + p_{i}^{n - 1}} )} & {{IF}\quad( {{\min( {p_{i}^{n},p_{i}^{n - 1}} )} < {\alpha\quad{AND}\quad\max( {p_{i}^{n},p_{i}^{n - 1}} )} > \beta} )} \\0 & {else}\end{matrix} $

The intermediate discrepancy function will only take into account anydelay, wherein the power of a certain delay has substantially changed,i.a. increased or decreased from the previous to the current power delayprofile. Therefore, the discrepancy function is given a valueproportional to the delay power if a certain delay power of any of thecurrent or the corresponding previous power delay profile is less than afirst power level α and any of the current or the corresponding previouspower delay profile is larger then a second power level β. Said powerlevels α, β have to be thoroughly tested an evaluated in each individualimplementation. A suitable level for the first power level α may e.g. bethe noise level and a suitable level for the second power level β maye.g. be twice the noise level (2α). This is an indication that amulti-path signal either has emerged or vanished. Consequently, onlybirths and deaths of multi-path signals are taken into account, whichmakes this embodiment more robust. Further, the power delay profilediscrepancy is computed by adding each value of the intermediatediscrepancy function to the power delay profile discrepancy variable,according to the same principles as described above, i.a.:${\Delta^{(n)} = {\sum\limits_{i = 1}^{N_{p}}{f( {p_{i}^{n} + p_{i}^{n - 1}} )}}},$

wherein N_(p) is the number of possible delays in each delay window.

In step 404, the power delay profile discrepancy variable is filtered tostabilize noise according to the same principles as set out above, i.a.:Δ_(filt)^((n)) = λΔ_(filt)^((n − 1)) + (1 − λ)Δ^((n)).

Finally, in step 405 the time lag τ^((n)) to the next path-searcheractivation is set using the filtered power delay profile discrepancyvariable and the penalty function penalizing large discrepancy valuesand rewarding low discrepancy values, according to above:τ^((n)) = f(Δ_(filt)^((n))).When the time lag is computed in step 405, the scheduling is returned tostep 400, where the path-searcher is activated with the computed timelag τ^((n)).

In an alternative embodiment, the mobile terminal 1 may receive signalsfrom several transmitters. This will require several path-searcheractivation time lags for each transmitter. However, different time lagsare not necessary, and a reduction of complexity is to schedule eachpath-searcher with the smallest time lag τ^((n)) between two consecutiveactivations. A consequence of this is, however, an increase of powerconsumption as the scheduling is not optimal.

In all the above embodiments, each value of the power delay profiles hasto be stored in a memory 12 of the mobile terminal 1 to be processed bye.g. a CPU (13) of the mobile terminal 1 according to the steps of themethod. Also, the processor is adapted to provide a filtering means forperforming the processing according to step 104. The path-searcher 11 isdescribed as being a part of the receiver 10. However, as is understood,the path-searcher 11 may equally be provided as a separate component.

The present invention has been described above in reference to a numberof embodiments. These are only for illustration purposes and should notbe considered as limiting the scope of the invention, which is bestdefined by the enclosed independent claims.

1. A method for scheduling activation of a path-searcher (11) for amobile communication terminal (1) capable of receiving multi-pathsignals originating from scattered signals transmitted from at least onetransmitter of a telecommunication system, characterized by the stepsof: deriving a first and a second power delay profile, each profilecomprising delay powers of a signal at different delays from theactivation of the path-searcher (11); computing a value of the powerdelay profile discrepancy between said first and second power delayprofiles; and computing a time lag to the next path-searcher activation,which is inverse proportional to said value of the power delay profilediscrepancy:
 2. The method according to claim 1, wherein thepath-searcher (11) is activated during a delay window comprising anumber of possible delays, to derive a power delay profile comprising atleast a subset of the powers of the delays received during thepath-searcher activation.
 3. The method according to claim 1 or 2,wherein the scheduling of the path-searcher activation is initialized bysetting the time lag to the next path-searcher activation to the minimumtime lag, τ^(min), if no previous power delay profile is retrievable. 4.The method according to claim 3, wherein the path-searcher (11) isactivated to derive the powers of signals received at a number of delaysduring the path-searcher activation, which powers forms the power delayprofile, and wherein the discrepancy between at least a subset of thedelays of the second power delay profile and the corresponding delays ofthe first power delay profile is computed to derive the value of thepower delay profile discrepancy.
 5. The method according to claim 3,wherein the path-searcher (11) is activated to derive the powers ofsignals received at a number of delays during the path-searcheractivation; a subset of said powers are selected to form a currentreduced power delay profile, wherein all powers present in the currentbut not in a previous reduced power delay profile is summarized toderived the value of the power delay profile discrepancy.
 6. The methodaccording to claim 3, wherein the path-searcher (11) is activated toderive the powers of signals received at a number of delays during thepath-searcher activation; a subset of said powers are selected to form acurrent reduced power delay profile comprising powers of differentdelays, which subset of powers is ranked and assigned a weight to form aranked current reduced power delay profile; the discrepancy between theweights of the delays of the ranked current reduced power delay profileand the weights of the corresponding delays of a ranked previous reducedpower delay profile is computed to derive the value of the power delayprofile discrepancy.
 7. The method according to claim 3, wherein thepath-searcher (11) is activated to derive the powers of signals receivedat a number of delays during the path-searcher activation; at least asubset of the powers are selected to form a current power delay profilecomprising powers of different delays; a value of the power of eachdelay of the current power delay profile and the corresponding previouspower delay profile are added to the power delay profile discrepancy ifat least one of said powers is below a first power level (α) and atleast one of said powers is above a second power level (β), otherwisesaid powers are discarded, wherein the value of the delay power profilediscrepancy is derived.
 8. The method according to claim 7, wherein thefirst power level (α) corresponds to the noise floor and the secondpower level (β) corresponds to twice the noise floor.
 9. The methodaccording to any of the claims 4-6, wherein the power delay profilediscrepancy is filtered to reduce the noise of the power delay profilediscrepancy, which filtering forms a filtered value of the power delayprofile discrepancy.
 10. The method according to claim 9, wherein thetime lag to the next path-searcher activation is given by a penaltyfunction of the filtered value of the power delay profile discrepancy,penalizing large power delay profile discrepancies and rewarding lowpower delay profile discrepancies.
 11. The method according to claim 10,wherein the penalty function is a piece-wise linear function, which setsthe time lag to the next path-searcher activation to the maximum timelag, τ_(max), if the filtered power delay profile discrepancy is smallerthan a first threshold value (δ₁), and which sets the time lag to thenext path-searcher activation to the minimum time lag, τ_(min), if thefiltered power delay profile discrepancy is larger than a secondthreshold value (δ₂).
 12. The method according to claim 11, wherein themaximum time lag is about 500 ms, and the minimum time lag is about 70ms.
 13. The method according to any of the previous claims, whereindifferent time lags are provided for activating several path-searchersif the receiver (10) of the mobile communication terminal (1) receivesmulti-path signals from several transmitters.
 14. A mobile terminalhaving an receiver (10) for receiving signals, a processor (12), and apath-searcher (11) for finding multi-path signal delays, characterizedin that the mobile terminal (1) is adapted to: derive a first and asecond power delay profile comprising powers of a signal at differentdelays; compute a value of the power delay profile discrepancy betweensaid first and second power delay profiles; and compute a time lag tothe next path-searcher activation, which is inverse proportional to saidvalue of the power delay profile discrepancy.
 15. The mobile terminalaccording to claim 14, wherein the mobile terminal (1) is furtheradapted to receive signals at different delays during a path-searcheractivation, and wherein the path-searcher (11) is adapted to compute thepowers of at least a subset of said delays to provide a power delayprofile.
 16. The mobile terminal according to claim 15, wherein themobile terminal comprises a memory for temporarily storing at least onepower delay profile.
 17. The mobile terminal according to any of theclaims 14-16, wherein the mobile terminal (1) further comprises afiltering means for filtering said power delay profile discrepancy. 18.The mobile terminal according to any of the claims 14-17, wherein theprocessor (12) of the mobile terminal (1) is adapted to compute thevalue of the power delay profile discrepancy between two consecutivepower delay profiles, and compute the time lag to the next path-searcheractivation based on said discrepancy and a penalty function penalizinglarge discrepancy values and rewarding low discrepancy values.
 19. Themobile terminal according to any of the claims 14-18, wherein thepath-searcher (11) is comprised in the receiver (10) of the mobileterminal (1).
 20. The mobile terminal according to any of the claims14-18, wherein the path-searcher (11) is provided as a separatecomponent of the mobile terminal (1).
 21. The mobile terminal accordingto any of the claims 14-20, wherein the mobile terminal (1) is a mobiletelephone.
 22. The mobile terminal according to any of the claims 14-20,wherein the mobile terminal (1) is a pager, a an electronic organizer,or a smartphone.
 23. The mobile terminal according to any of the claims14-22, wherein the mobile terminal (1) is further adapted to select anumber of delays, {circumflex over (N)}_(p), of the power delay profilehaving the highest powers.
 24. The mobile terminal according to claim23, wherein the mobile terminal is further adapted to assign a weight toeach delay power of the power delay profile.